Common-mode insensitive sampler

A method for rejecting common-mode input voltage variations in a differential sampler comprising:

• sampling a first differential input voltage, Vinp, to provide a first sampled input voltage having a first common mode component;
• sampling a second differential input voltage, Vinm, to provide a second sampled input voltage having a second common mode component;
• sensing common-mode variations in the first and second sampled input voltages, to detect such common-mode variations as a corresponding common mode voltage Vcmi;
• sampling the common mode voltage Vcmi to provide a sampled common mode voltage;
• discharging current resulting from the first and second sampling steps via a common discharge node, Vdch;
• and
• maintaining the discharge node, Vdch, at a predetermined constant voltage difference with respect to the common mode voltage, Vcmi.

An apparatus for rejecting common-mode input variations in a differential sampler, comprising:

• a first sampler, for sampling a first differential input voltage at a terminal Vinp, at a first given time, the first differential input voltage having a first common mode component;
• a first sampler switch, connected between the first differential input voltage terminal, Vinp, and a first sampler node capacitive element of capacitance Cs;
• a first discharge switch connected between the first sampler node and a common discharge node, Vdch;
• a second sampler comprising a second sampler switch, connected between a second differential input voltage terminal, Vinm, and a second sampler node capacitive element of capacitance Cs;
• a second discharge switch connected between the second sampler node and the common discharge node, Vdch;
• a first resistive element connected between the first differential input terminal and a common node, Vcmi;
• a second resistive element connected between the second differential input terminal and the common node, Vcmi;
• the second sampler, for sampling a second differential input voltage, Vinm, at the given time, the second differential input voltage having a second common mode component;
• a third sampler, comprising a third sampler switch, connected to sample the common mode voltage, Vcmi, at the given time and storing the same on a respective sampling capacitive element, Ccmc;
• and
• a switch control circuit for (a) obtaining samples of the first and second differential input terminals and the common node Vcmi by controlling the first, second and third sampler switches to operate at the same time according to a first clock signal, Ktp, and (b) discharging energy stored in both of the capacitive elements Cs via a common path by operating the first and second discharge switches according to a second clock signal, Kct, so that voltage at the common discharge node Vdch is held at a constant difference with respect to voltage on the common node Vcmi by capacitive element Ccmc.